Corrosion resistant material

ABSTRACT

The invention relates to a material with high corrosion resistance in media with high chloride concentration, suitable for equipment in oil-field technology. According to the invention, for making a paramagnetic material with high yield strength, high notched impact strength, high a fatigue strength under reversed stresses and a low ductile transition temperature with concomitant improved high corrosion resistance, in particular, resistance to hole corrosion there is provided a material consisting essentially of the elements in wt. %: carbon (C) less than/equal to 0.03; silicon (Si) less than/equal to 0.89; manganese (Mn) 0.51 to 4.49; chromium (Cr) 25.1 to 38.9; molybdenum (Mo) 2.1 to 5.9; nickel (Ni) 22.9 to 38.9; copper (Cu) 0.51 to 1.49; nitrogen (N) 0.17 to 0.19; iron (Fe) the remainder, along with impurities arising during production. Said material is hot formed in a state free of nitride precipitates and other precipitated associated phases and, after cooling, is cold formed in a ferrite-free state and has a permeability of less than 1.0048; a yield strength (R p02 ) greater than 710 N/mm 2 ; a notched impact strength of over 60 J; a fatigue strength under reversed stresses of at least ±310 N/mm  2 , where N=10 7  load reversals and a fracture appearance transition temperature of less than −28° C. (FATT).

[0001] The invention relates to a material with a high corrosion resistance in media with a high chloride concentration, suitable for equipment in oilfield technology, in particular for drilling line components, comprising the elements carbon (C), silicon (Si), manganese (Mn), chromium (Cr), molybdenum (Mo), nickel (Ni), copper (Cu), nitrogen (N), iron (Fe) and contaminants due to manufacture, which material is hot formed and, after cooling, is cold formed.

[0002] Corrosion resistant materials which show paramagnetic behavior and feature a high degree of strength, can be used for equipment in oil field technology, particularly for drilling line components. However, higher demands are always being made on the parts and stricter standards are always being set for the materials.

[0003] In order to be able to conduct directional measurements during the sinking or boring of a drill-hole with the necessary precision, the material must have a permeability of less than 1.005.

[0004] A high mechanical strength, in particular a high 0.2% elongation value, is necessary in view of an advantageous design in terms of industrial engineering and of high operational safety of the parts, because it is intended to stress same up to the limiting values of the respective material load capacity, and because increasingly large drilling depths are required. Furthermore, a notched impact strength of the material is important, because the parts often have to withstand high stresses in the form of impacts or shocks.

[0005] A high fatigue strength under reversed stresses is important in many cases, in particular for drilling line parts and drill stems, because increasing or changing stresses can be present during a rotation of the parts or of the drill stems, respectively.

[0006] The parts are often installed or used at low temperatures so that the fracture appearance transition temperature (FATT) of the material also plays an important role.

[0007] The corrosion behavior for parts used in oilfield technology is of crucial importance, that is, on the one hand stress corrosion cracking (SCC) and on the other pitting corrosion (pitting, CPT).

[0008] As shown by the above statements, materials which have a high degree of corrosion resistance in media with a high chloride concentration and are suitable for equipment in oilfield technology are simultaneously exposed to a plurality of high stresses.

[0009] The object of the invention is to provide a paramagnetic material with a high yield strength, high notched impact strength and high fatigue strength under reversed stresses as well as a low fracture appearance transition temperature, which at the same time is corrosion-resistant, in particular resistant to pitting, in chloride-containing media.

[0010] This object is attained with a material of the type mentioned at the outset by this consisting essentially of the elements in percent by weight Carbon (C) less than/equal to 0.03 Silicon (Si) less than/equal to 0.89 Manganese (Mn) 0.51 to 4.49 Chromium (Cr) 25.1 to 38.9 Molybdenum (Mo) 2.1 to 5.9 Nickel (Ni) 22.9 to 38.9 Copper (C) 0.51 to 1.49 Nitrogen (N) 0.17 to 0.29 Iron (Fe) balance

[0011] and contaminants due to manufacture, which material is hot formed in a condition free of nitride precipitates and without precipitated associated phases and, after a cooling, cold formed in a condition free of ferrites, and having

[0012] a permeability of less than 1.0048

[0013] a yield strength (R_(p 0 2)) of more than 710 N/mm²

[0014] a notched impact strength of over 60 J

[0015] a fatigue strength under reversed stresses of more than ±310 N/mm²

[0016] at N=10⁷ load reversal and

[0017] a fracture appearance transition temperature of less than −28° C. (FATT).

[0018] The advantages achieved by the invention lie in particular in the alloying technology effect of a balanced nitrogen concentration. Surprisingly, it was found that a particularly high output can be achieved in the manufacture of parts. Although there cannot be any nitride precipitates with a hot forming, the forming property of the material at a varying forging temperature is abruptly impaired at contents of over 0.29 percent by weight nitrogen. In the narrow concentration range of 0.17 to 0.29 percent by weight N a precipitation of associated phases can also be easily prevented if the other alloying elements are present in the provided content ranges. Nitrogen, nickel and molybdenum thereby also synergistically produce an extremely high resistance to pitting.

[0019] At 0.03 percent by weight, the carbon content of the alloy has an upper limit for corrosion chemistry reasons, with a further reduction thereof increasing the corrosion resistance of the material, in particular pitting and stress corrosion cracking.

[0020] The silicon content in the material according to the invention should not exceed 0.89 percent by weight for corrosion chemistry reasons and in particular because of the low magnetic permeability.

[0021] The nitrogen solubility of the alloy and the austenite stabilization are promoted by manganese. However, to prevent pitting, the manganese contents must have an upper limit of 4.49 percent by weight with nickel being added to the alloy instead. A minimum content of 0.51 percent by weight manganese is necessary for an effective sulfur binding.

[0022] One of the particularly important alloying elements with regard to corrosion resistance is chromium, because chromium is the basis for forming a passive layer on the surface of the parts. Contents of at least 25.1 percent by weight Cr are necessary in synergistic effect with the other alloying elements, in particular Mo and N, in order to largely prevent a possible piercing of this layer in places. With contents higher than 38.9 percent by weight the danger of a precipitation of intermetallic phases increases.

[0023] Although the alloying element molybdenum is extremely important for a resistance of the material to crevice and pitting corrosion, the content should not exceed 5.9 percent by weight, because then there is a sudden increased tendency to form associated phases. Contents lower than 2.1 percent by weight impair the corrosion behavior of the material disproportionally.

[0024] The alloying element nickel is important in the provided concentrations for stabilizing the cubic face-centered atomic lattice, thus for low permeability, and interacting with chromium and molybdenum it is effective for avoiding pitting corrosion. Up to 38.9 percent by weight, the toughness, the FATT and the fatigue strength under reversed stresses are advantageously increased. If it falls below 22.9 percent by weight, the stabilizing effect regarding corrosion, in particular stress corrosion cracking, is reduced to an increasing extent in chloride-containing media and with respect to the magnetic values in cold working; thus there is an increased tendency to form zones with strain-induced martensite.

[0025] A copper content within the limits of the alloy is also provided to increase corrosion resistance, even though the effect of this element has occasionally been questioned.

[0026] As mentioned earlier, the nitrogen content is synergistically adapted to the remainder of the alloy composition. This content of 0.17 to 0.29 percent by weight has the further advantage that a block can be left to solidify under atmospheric pressure without gas bubbles being formed therein by exceeding the solubility limit during solidification.

[0027] The magnetic, the mechanical and in particular the corrosion resistance values of the material can be set at a particularly high level, if it consists essentially of the elements in percent by weight: C = less than/equal to 0.02, preferably 0.005 to 0.02 Si = less than/equal to 0.75, preferably 0.20 to 0.70 Mn = 1.1 to 2.9., preferably 2.01 to 2.6 Cr = 26.1 to 27.9, preferably 26.5 to 27.5 Mo = 2.9 to 5.9, preferably 3.2 to 3.8 Ni = 27.9 to 32.5, preferably 30.9 to 32.1 Cu = 0.98 to 1.45, preferably 1.0 to 1.4 N = 0.175 to 0.29, preferably 0.18 to 0.22

[0028] High mechanical property values at a relative magnetic permeability of 1.004 and below are achieved when the material is hot formed at least 3.6-fold in a condition free of precipitates and is cold formed at a temperature of 100 to 590° C., preferably 360 to 490° C., with a degree of forming of less than 38%, preferably 6 to 19%. According to the invention the material features a pitting corrosion potential in a neutral solution at room temperature of more than 1,100 mVH/1,000 ppm chlorides and/or 1,000 mVH/80,000 ppm chlorides.

[0029] The invention is explained in more detail using examples.

[0030] Table 1 shows the chemical composition of the alloys according to the invention and the comparison materials. The characteristic values for hot forming and cold forming the forged pieces can also be taken from this table.

[0031] The magnetic and the mechanical characteristic values of these materials can be taken from Table 2.

[0032] Table 1 lists the comparison alloys with the sample identifiers 1 through 5, and the alloys composed according to the invention with the sample identifiers A through E. The test results of the materials can be taken from Table 2. These results will be discussed briefly below.

[0033] The alloys 1 through 3 have low nitrogen contents, and therefore show no desired hardening during a cold forming, as revealed by the R_(P0.2) values, and low numerical values of ±270, 210 and 290 N/mm² were also ascertained for the fatigue strength under reversed stresses (not given in the table).

[0034] In corrosion chemistry terms neither the SCC nor the CPT values are adequate, which can be attributed in particular to the respective low Mo contents and, in the case of material 2, to a low Cr content.

[0035] The alloys 4 and 5 have a not sufficiently high and an excessive nitrogen concentration, which leads to higher yield point values and also increases the value for the fatigue strength under reversed bending stresses (±308, 340 N/mm²). Due to a low Cr content, there is a disadvantageous DUAL microstructure (etching on the grain boundaries) in material 4, and it should be further noted that, despite adequate Mo concentrations due to the lower Cr contents, material 5 does not meet the requirements for corrosion-resistance, either. The results for alloys A through E show that the nitrogen contents lead to a desired hardening by a cold forming, and the respective concentrations of nitrogen, nickel and molybdenum synergistically give rise to a high corrosion resistance of the material in chloride-containing media, in particular a high resistance to pitting. TABLE 1 1. Step/Hot Forming 2. Step Chemical Composition Degree of Forming Forming Forming Sample C Si Mn Cr Ni Mo Cu N Forming (-fold) Temp. [° C.] Cooling (%) Temp. [° C.] 1 0.02 0.31 1.92 27.20 30.66 0.30 0.60 0.02 4.5 1050/980 air 15 450 2 0.05 0.40 1.30 17.52 10.20 0.05 0.05 n.d. 5.0 1070/910 water n.d. n.d. 3 0.025 0.41 2.51 25.28 28.07 0.35 n.d. 0.08 5.2 1050/900 air 18 460 A 0.03 0.35 1.81 26.60 28.52 3.31 1.24 0.18 5.0 min. 850 water 15 480 B 0.025 0.28 2.25 27.44 34.58 3.78 1.30 0.21 5.8 min. 850 water 20 470 C 0.02 0.30 1.10 27.28 31.20 5.12 1.05 0.20 5.5 min. 850 water 18 470 D 0.025 0.28 1.60 30.56 35.38 2.20 0.70 0.28 5.2 min. 850 water 15 450 E 0.02 0.30 2.61 27.10 29.32 2.71 0.62 0.29 5.0 min. 850 water 20 480 4 0.01 0.6 1.7 17.30 13.20 2.7 0.01 0.16 5.0 1080/950 air 8 350 5 0.02 1.4 0.8 23.50 15.36 1.4 0.01 0.30 4.8 n.d. n.d. n.d. n.d.

[0036] TABLE 2 Toughness Rel. Magn. (ISO-V) Oxalic Pitting Permeability R_(P0.2) Rm 20° C. FATT Acid Test SCC CPT Sample [μr] [N/mm²] [N/mm²] [Joule] [° C.] ASTM-A262 45% MgCl₂ 22% NaCl 1 1.003 470 780 150 −45 STEP 200 MPa/min. 720^(h) max. 5° C. 2 1.002 430 750 170 −50 STEP 100 MPa/min. 8^(h ) max. 5° C. 3 1.003 560 790 160 −50 STEP 150 MPa/min. 720^(h) max. 5° C. A 1.002 930 1050 140 −45 STEP 450 MPa/min. 720^(h) 55° C. B 1.003 1010 1110 120 −45 STEP 550 MPa/min. 720^(h) 60° C. C 1.003 940 1040 107 −40 STEP 650 MPa/min. 720^(h) 85° C. D 1.003 980 1090 99 −35 STEP 600 MPa/min. 720^(h) 65° C. E 1.002 1000 1150 130 −45 STEP 450 MPa/min. 710^(h) 65° C. 4 1.005 670 820 130 −40 DUAL 100 MPa/min. 720^(h) 15° C. 5 1.001 810 910 120 −45 STEP 150 MPa/min. 720^(h) 35° C. 

1. Material with a high corrosion resistance in media with a high chloride concentration, suitable for equipment in oilfield technology, in particular for drilling line components, consisting essentially of the elements in percent by weight: Carbon (C) less than/equal to 0.03 Silicon (Si) less than/equal to 0.89 Manganese (Mn) 0.51 to 4.49 Chromium (Cr) 25.1 to 38.9 Molybdenum (Mo) 2.1 to 5.9 Nickel (Ni) 22.9 to 38.9 Copper (Cu) 0.51 to 1.49 Nitrogen (N) 0.17 to 0.29 Iron (Fe) balance

and contaminants due to manufacture, which material is hot formed in a condition free of nitride precipitates and without precipitated associated phases and, after a cooling, cold formed in a condition free of ferrites, and having a permeability of less than 1.0048 a yield strength (R_(p)0 2) of more than 710 n/mm² a notched impact strength of over 60 J a fatigue strength under reversed stresses of at least ±310 n/mm² at N=10⁷ load reversal and a fracture appearance transition temperature of less than −28° C. (FATT):
 2. Material according to claim 1, consisting essentially of the elements in percent by weight: C = less than/equal to 0.02, preferably 0.01 to 0.02 Si = less than/equal to 0.75, preferably 0.20 to 0.70 Mn = 1.1 to 2.9, preferably 2.01 to 2.6 Cr = 26.1 to 27.9, preferably 26.5 to 27.5 Mo = 2.9 to 5.9, preferably 3.2 to 3.8 Ni = 27.9 to 32.5, preferably 30.9 to 32.1 Cu = 0.98 to 1.45, preferably 1.0 to 1.4 N = 0.175 to 0.29, preferably 0.18 to 0.22


3. Material according to claim 1 or 2, which, as is known per se, is hot formed at least 3.6-fold in a condition free of precipitates, and cold formed at a temperature of 100 to 590° C., preferably 360 to 490° C., with a degree of forming of less than 38%, preferably 6 to 19%.
 4. Material according to one of claims 1 to 3, having a pitting potential in a neutral solution at room temperature of more than 1,100 mVH/1,000 ppm chlorides and/or 1,000 mVH/80,000 ppm chlorides. 